Plants obtain essential heavy metals, such as Zn, Ni, and Cu, by absorbing metal ion substrates from their environment by various transport mechanisms mediated by transmembrane transporters expressed on the surface of root cells and other vascular tissues. Transporters classified as P-type ATPases, such as P1B-type ATPases, are transporters that translocate positively charged substrates across plasma membranes by utilizing energy liberated from exergonic ATP hydrolysis reactions. P1B-type ATPases are also referred to as heavy metal ATP-ases (“HMAs”) or CPx-type ATPases. HMAs have been grouped by substrate specificity into two subclasses, the Cu/Ag and Zn/Co/Cd/Pb groups. The first P1B-type ATPase to be characterized in plants is AtHMA4, cloned from Arabidopsis. Substrate selectivity by HMAs is not strictly limited to the transport of essential metals in that several non-essential metals can be recognized indiscriminately as substrates, resulting in the accumulation of many non-essential metals, such as Cd, Pb, As, and Hg.